Construction method of ultra-small clear distance double-hole separation type loess tunnel

Abstract

The invention belongs to the technical field of tunneling in loess areas, and particularly relates to an ultra-small clear distance double-hole separation type loess tunnel construction method. A temporary plugging structure is constructed on the end wall of a tunnel changed from a large hole to a left small hole and a right small hole, the left small hole and the right small hole are excavated in a staggered mode, middle soil clamping columns are supported when the left small hole and the right small hole are excavated, the small holes are excavated firstly, middle soil clamping column side radial grouting + large arch springing reinforcement + tunnel bottom grouting reinforcement is adopted, and then the small holes are excavated, middle soil clamping column side compaction grouting + opposite-pulling anchor rod locking primary supporting reinforcement is adopted; a double-hole staggered synchronous construction process is adopted, and the modes of large-span section surrounding rock reinforcement treatment, large-hole end wall temporary plugging, ultra-small clear distance loess tunnel middle rock pillar stabilization reinforcement treatment and the like are adopted, so that double-hole separated construction is achieved, personnel investment is reduced, the construction period is shortened, the construction efficiency is improved, the construction cost is reduced, and the construction quality is guaranteed; and the construction safety is ensured.

Description

Technical Field

[0001] The invention belongs to the technical field of tunnel excavation in loess areas, and in particular relates to a construction method for an ultra-small clearance double-hole separated loess tunnel. Background Art

[0002] During the construction of loess tunnels, there are problems such as large and difficult-to-control settlement, strong loess collapsibility, high surrounding rock pressure during construction, and easy cracking of the support. There are many engineering cases for the construction of ultra-large span and small clearance tunnels, but there are few construction cases for loess bifurcated tunnels with ultra-large spans and ultra-small clearances. The tunnel clearance is small, and the tunnel disturbance is obvious during the excavation of the two tunnels. The stability requirements of the soil column between the two tunnels are high. The surrounding rock is greatly affected by the excavation, which has a great impact on the internal force and deformation of the tunnel support. Summary of the invention

[0003] An object of the present invention is to solve at least the above problems and to provide at least the advantages which will be described hereinafter.

[0004] The present invention provides the following technical solutions: a method for constructing an ultra-small clearance double-hole separated loess tunnel, in which a temporary blocking structure is constructed on the end walls of the left and right small holes converted from the large hole of the tunnel, the left and right small holes are excavated in a staggered manner, and the middle earth column is supported when the left and right small holes are excavated. The small hole is excavated first, and radial grouting of the middle earth column side + large arch foot reinforcement + tunnel bottom grouting reinforcement is adopted. The small hole is excavated later, and compaction grouting of the middle earth column side + tension anchor locking initial support reinforcement is adopted.

[0005] Furthermore, the construction method of the ultra-small clearance double-hole separated loess tunnel comprises the following steps performed in sequence:

[0006] Step S1: Perform initial support reinforcement of the tunnel before the left and right small tunnels enter the tunnel, including external reinforcement and internal reinforcement; external reinforcement includes treatment of surface gullies, surface caves, and surface cracks; internal reinforcement includes radial grouting and arch construction;

[0007] Step S2: constructing a temporary blocking structure at the end walls of the left and right small tunnels of the tunnel;

[0008] Step S3: First, dig a small hole for tunnel construction, excavate support construction, invert and filling construction, and secondary lining construction; during the excavation and support construction stage, radial grouting of the middle soil column side, reinforcement of the large arch foot, and grouting reinforcement of the tunnel bottom are carried out;

[0009] Step S4: post-excavation small hole entry construction, excavation support construction, lining construction; during the excavation support construction stage, side compaction grouting of the middle sandwich soil column and locking of the tension anchor rod for initial support reinforcement;

[0010] Step S5: dismantling the arch and constructing the secondary lining of the left and right small holes;

[0011] Step S5: Construction of reinforced concrete structure of the end walls of the left and right small tunnels;

[0012] Step S6: The left and right small holes are excavated synchronously.

[0013] Furthermore, in step S1, the surface gully treatment includes surface survey, gully replacement, laying of wire mesh, and mortar plastering;

[0014] Surface cave management includes surface exploration, cave backfilling and compaction;

[0015] Surface crack management includes surface exploration, crack excavation, crack replacement and compaction.

[0016] Further, in step S1, radial grouting during tunnel reinforcement includes: arranging grouting pipes along the side walls and arch top of the tunnel, reinforcing the arch ring 5m outside the designed excavation contour line of the tunnel, the length of the grouting pipe is 5-7m, and the grouting holes are arranged in a plum blossom shape around the pipe wall. The grouting material is cement / water glass double slurry, and the initial grouting pressure is 0.5-1.0MPa;

[0017] The construction scope of the sleeve arch is within 9m outside the end wall of the tunnel where the large tunnel becomes a double tunnel. The arch frame spacing of the sleeve arch is 1.2m. When the arch frame is installed, φ42 locking foot anchor pipes are set, with two locking foot anchor pipes at each location, and the length of the locking foot anchor pipe is 4m; the longitudinal connection of the arch frame is welded with steel bars; after the sleeve arch is installed, C25 concrete is sprayed within 50cm before and after the arch frame, with a spraying thickness of 30cm.

[0018] Furthermore, in step S3, the radial grouting range on the side of the middle soil column in the first excavated small hole is from the arch top close to the side of the small hole to be excavated later to the base of the invert close to the side of the small hole to be excavated later, and the end of the grouting pipe in the middle soil column range penetrates into the excavation surface of the small hole to be excavated later by not less than 50 cm; the arch frame spacing in the first excavated small hole is increased to 0.5m, and a large arch foot is set at the bottom of the arch frame on the side of the middle soil column.

[0019] Further, in step S4, the compaction grouting of the soil column side in the later excavated small hole includes reserving a grouting pipe for the initial support of the later excavated small hole, and performing compaction grouting on the back of the initial support after the initial support of the later excavated small hole is completed; the compaction grouting range is from the spandrel close to the side of the first excavated small hole to the arch foot close to the side of the first excavated small hole; the grouting material is cement plus HPC admixture single liquid slurry;

[0020] The tension anchor locking initial support reinforcement includes that when the thickness of the middle soil column is less than 2m, the grouting pipe in the first excavated small hole is respectively welded to the arch frame in the first excavated small hole and the arch frame in the later excavated small hole by connecting steel plates, which plays the role of tension anchor.

[0021] Furthermore, in step S2, the temporary blocking structure for constructing the end walls of the left and right small holes in the tunnel includes a blocking structure outside the excavation range of the left and right small holes in the tunnel large hole and a supporting structure within the excavation range of the left and right small holes; along the tunnel excavation direction from inside to outside, the blocking structure includes a drainage system, a steel support structure, a reinforced anchor rod, a first steel mesh and a first shotcrete layer in sequence; the supporting structure includes a second shotcrete layer, anchor rods and a second steel mesh.

[0022] Furthermore, the steel support structure includes an outer support ring on the contour line of the large tunnel hole and an inner support ring on the contour lines of the left and right small tunnel holes, the outer support ring and the inner support ring are connected by a steel frame, and the outer support ring, the inner support ring and the steel frame are located in the same plane;

[0023] The steel frame includes main vertical supports, transverse supports and secondary vertical supports. The main vertical supports are located between the left and right small holes. A group of locking foot anchor pipes are installed at the bottom of the main vertical supports. The locking foot anchor pipes are welded to the main vertical supports with L-shaped steel bars. The secondary vertical supports are symmetrically arranged on both sides of the main vertical supports. A complete transverse support is installed in each width section of the steel frame, and the transverse support is connected to the main vertical supports, secondary vertical supports, outer support rings or inner support rings.

[0024] Furthermore, 3:7 lime soil was used for gully replacement and crack replacement, and M10 mortar was used for mortar finishing during surface gully treatment.

[0025] Furthermore, the arch frame in the later excavated small hole is longitudinally connected with a steel plate on a side close to the first excavated small hole.

[0026] Compared with the prior art, the advantages of the present invention are:

[0027] 1. The double-tunnel staggered synchronous construction technology is adopted. Through the reinforcement of the surrounding rock of the large-span section, the temporary blocking of the end wall of the large tunnel, and the reinforcement of the stable rock column in the middle of the ultra-small clear distance loess tunnel, the double-tunnel separate construction is realized, which reduces the input of personnel, shortens the construction period, improves the construction efficiency, reduces the construction cost, ensures the construction quality and ensures the construction safety.

[0028] 2. A reinforcement method for the excavation and support of the left and right small holes was invented, that is, the right hole (excavated first) adopts the left radial grouting + large arch foot reinforcement + tunnel bottom grouting method, and the left hole (excavated later) adopts the right compaction grouting + tension anchor locking initial support method, which improves the stability of the middle soil column and ensures the safety of the small clear distance double-hole tunnel construction.

[0029] 3. Improved construction efficiency. The traditional middle-hole construction method takes 1.7 days for excavation and support per linear meter, while this method takes 0.56 days for excavation and support per linear meter. The excavation and support efficiency is increased by 67%, which effectively improves the construction efficiency of small clear distance sections, shortens the construction period of small clear distance sections, speeds up the progress of the project, and relieves the pressure of the construction period. It has significant economic and social benefits.

[0030] 4. Improve the construction quality. Compared with the traditional middle tunnel construction method, this method uses independent closure of the left and right tunnel structures, with reasonable structural stress and good durability. The lining waterproofing is applied once, with good waterproofing effect. The second lining is integrally cast and formed, with good appearance quality, ensuring the overall construction quality of the tunnel. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A bird's-eye view of the tunnel's large hole changing into small holes on the left and right;

[0032] Figure 2 This is a schematic diagram of the radial grouting and compaction grouting range of the middle soil column;

[0033] Figure 3 is a schematic diagram of the steel support structure;

[0034] Figure 4 This is a side view of the temporary blocking structure at the end walls of the left and right small holes where the large tunnel hole becomes smaller.

[0035] In the figure: 1-tunnel main hole; 2-left hole; 3-right hole; 4-middle soil column; 5-end wall of left and right small holes transformed from the tunnel main hole; 6-initial support of left hole; 7-initial support of right hole; 8-compression grouting pipe; 9-radial grouting pipe of right small hole; 10-steel support structure; 10.1-outer support ring; 10.2-inner support ring; 10.3-horizontal brace; 10.4-main vertical brace; 10.5-secondary vertical brace; 11-reinforcement anchor rod; 12-first steel mesh; 13-first shotcrete layer; 14-drainage system. DETAILED DESCRIPTION

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0037] like Figure 1 , Figure 2 As shown: a method for constructing a double-hole separated loess tunnel with ultra-small clearance, a temporary blocking structure is constructed at the end wall of the left and right small holes of the tunnel, the left and right small holes are staggered for excavation, and the middle soil column is supported when the left and right small holes are excavated. The small hole is excavated first, and radial grouting of the middle soil column side + large arch foot reinforcement + tunnel bottom grouting reinforcement is adopted, and the small hole is excavated later, and the middle soil column side compaction grouting + tension anchor locking initial support reinforcement is adopted, which improves the stability of the middle soil column and ensures the safety of the small clearance double-hole tunnel construction. Combined with the attached drawings, in this embodiment, the small hole excavated first is the right hole, and the small hole excavated later is the left hole.

[0038] A method for constructing an ultra-small clearance double-hole separated loess tunnel comprises the following steps performed in sequence:

[0039] Step S1: Perform initial support reinforcement of the tunnel before the left and right small tunnels enter the tunnel, including external reinforcement and internal reinforcement; external reinforcement includes the treatment of surface gullies, surface caves, and surface cracks; internal reinforcement includes radial grouting and arch construction; after multiple reinforcements are achieved, the intrusion of surface water outside the tunnel is avoided, the tunnel is stable, the problem is solved from the source, and safety in the construction area is ensured;

[0040] Step S2: constructing a temporary blocking structure at the end walls of the left and right small tunnels of the tunnel;

[0041] Step S3: First, dig a small hole (right hole) for tunnel construction, excavation support construction, invert and filling construction, and secondary lining construction; during the excavation support construction stage, radial grouting of the middle soil column side, reinforcement of the large arch foot, and grouting reinforcement of the tunnel bottom are carried out;

[0042] Step S4: excavation of the small hole (left hole) at the rear, tunnel construction, excavation support construction, and lining construction; during the excavation support construction stage, side compaction grouting of the middle soil column and locking of the tension anchor rods for initial support reinforcement are performed;

[0043] Step S5: dismantling the sleeve arch and constructing the secondary lining of the left and right small holes;

[0044] Step S5: Construction of reinforced concrete structure of the end walls of the left and right small tunnels;

[0045] Step S6: The left and right small holes are excavated synchronously.

[0046] In step S1, the surface gully treatment includes surface survey, gully replacement, laying of wire mesh, and mortar plastering;

[0047] Surface cave management includes surface exploration, cave backfilling and compaction;

[0048] Surface crack management includes surface exploration, crack excavation, crack replacement and compaction.

[0049] 3:7 lime soil is used for gully replacement and crack replacement, and M10 mortar is used for mortar finishing during surface gully treatment.

[0050] In step S1, radial grouting during tunnel reinforcement includes: arranging grouting pipes along the side walls and arch top of the tunnel, reinforcing the arch ring 5m outside the designed excavation contour line of the tunnel, the grouting pipes are made of φ42 / 3.5mm steel pipes, the length of the grouting pipes is 5-7m, the grouting holes are arranged in a plum blossom shape around the pipe wall, the grouting material is cement / water glass double slurry, and the initial grouting pressure is 0.5-1.0MPa;

[0051] The construction scope of the sleeve arch is within 9m outside the end wall of the tunnel where the large tunnel becomes a double tunnel. The arch frame spacing of the sleeve arch is 1.2m. The arch frame is made of I25b I-beam. Before production, a processing drawing is drawn according to the scanned initial support section. When the arch frame is installed, φ42 locking foot anchor pipes are set, with two locking foot anchor pipes at each location, and the length of the locking foot anchor pipe is 4m; the longitudinal connection of the arch frame is welded with steel bars; after the sleeve arch is installed, C25 concrete is sprayed within 50cm before and after the arch frame, with a spraying thickness of 30cm.

[0052] like Figure 3 , Figure 4 As shown: in step S2, the temporary blocking structure for constructing the end walls of the left and right small holes in the tunnel hole includes the blocking structure outside the excavation range of the left and right small holes in the tunnel hole and the supporting structure within the excavation range of the left and right small holes; along the tunnel excavation direction from inside to outside, the blocking structure includes the drainage system 14, the steel support structure 10, the reinforcing anchor rod 11, the first steel mesh 12 and the first sprayed concrete layer 13 in sequence; the supporting structure includes the second sprayed concrete layer, the anchor rod and the second steel mesh.

[0053] The steel support structure 10 includes an outer support ring 10.1 on the contour line of the tunnel large hole and an inner support ring 10.2 on the contour lines of the left and right small holes. The outer support ring 10.1 and the inner support ring 10.2 are connected by a steel frame. The outer support ring 10.1, the inner support ring 10.2 and the steel frame are located in the same plane.

[0054] The steel frame includes a main vertical support 10.4, a transverse support 10.3 and a secondary vertical support 10.5. The main vertical support 10.4 is between the two small holes. A group of locking foot anchor pipes are installed at the bottom of the main vertical support 10.4. A group of locking foot anchor pipes includes 2 4m long φ42 locking foot anchor pipes. The locking foot anchor pipes are connected to the main vertical support 10.4 by welding L-shaped steel bars. The locking foot anchor pipes are connected to the main vertical support 10.4 at a distance of 60cm from the bottom. The secondary vertical supports 10.5 are symmetrically arranged on both sides of the main vertical support 10.4. The spacing between the secondary vertical supports 10.5 and the secondary vertical supports 10.5 and the main vertical supports 10.4 is 2m. A complete transverse support 10.3 is installed in each width section of the steel frame. The transverse support 10.3 is connected to the main vertical support 10.4, the secondary vertical support 10.5, the outer support ring 10.1 or the inner support ring 10.2, and the spacing between the transverse supports 10.3 is 1.2m.

[0055] The outer support ring 10.1, the inner support ring 10.2 and the main vertical support 10.4 are formed by splicing I-beams, steel plates are welded at both ends of the I-beams, and the I-beams are connected by bolts on the steel plates.

[0056] The secondary vertical brace 10.5 and the transverse brace 10.3 adopt a grid steel frame, which is made of main reinforcement (φ22 steel bars), cross-shaped reinforcement (φ14 steel bars) and 80*80 angle steel. The transverse brace 10.3 is embedded in the flange plates on both sides of the I-beam and is welded to the I-beam.

[0057] The reinforcing anchor rod 11 is a mortar anchor rod. The length of the reinforcing anchor rod 11 is 4m, the installation spacing is 1.2m*1.2m, the medicine bag of the reinforcing anchor rod 11 is 20-30S fast consolidation material, the anchor rod head of the reinforcing anchor rod 11 is an expansion shell anchor rod head, and the tail of the reinforcing anchor rod 11 is connected to the L-shaped steel bar through a threaded sleeve, and the L-shaped steel bar is welded to the steel support structure.

[0058] The drilling direction of the reinforcing anchor rod 11 should be as perpendicular as possible to the surrounding rock and the main structural surface of the rock formation. The reinforcing anchor rod 11 should be driven in and the rod body of the reinforcing anchor rod 11 should be rotated to fully stir the medicine bag until the reinforcing anchor rod 11 reaches the bottom of the hole and slurry flows out of the hole mouth.

[0059] The anchor rods are drug pack anchor rods with an installation spacing of 1.2m*1.2m and are arranged in a plum blossom shape.

[0060] The drainage system 14 includes vertical drainage blind pipes and horizontal drainage blind pipes. The vertical drainage blind pipes are φ90 double-wall perforated corrugated pipes, and the installation spacing of the vertical drainage blind pipes is 1.5m. The horizontal drainage blind pipes are φ50 single-wall perforated corrugated pipes, and the installation spacing of the horizontal drainage blind pipes is 3m. The horizontal drainage blind pipes are introduced into the vertical drainage blind pipes through connectors, and a bend is set at 1m at the bottom of the vertical drainage blind pipe to lead out water.

[0061] After the construction of the temporary blocking structure is completed, backfill and counterpressure are carried out. The backfill height reaches the step in the tunnel hole, the backfill length is 11m, and the backfill soil surface slope is 1:1.

[0062] In step S3 and step S4, the small hole is excavated using the step method, and the specific process is as follows:

[0063] 1. Excavation on the left side of the upper step, middle step, and right side of the lower step;

[0064] 1) Use the steel frame erected in the previous cycle to construct tunnel advance support;

[0065] 2) Excavate 1 steel frame on the upper step, 2 steel frames on the left side of the middle step, and 2 steel frames on the right side of the lower step;

[0066] 3) Carry out the initial support of the tunnel structure on the upper step, the left side of the middle step, and the right side of the lower step, that is, spray 4 cm thick concrete, hang steel mesh and erect steel frames, install locking anchor pipes, and drill system anchor rods;

[0067] 4) Re-spray concrete to designed thickness.

[0068] 2. Excavation of the upper step, the right side of the middle step, the left side of the lower step and the initial support of the invert;

[0069] 1) Excavate 1 steel frame for the upper step, 2 steel frames on the right side of the middle step, 2 steel frames on the left side of the lower step, and 2 steel frames for the initial support of the invert;

[0070] 2) Spray 4cm thick concrete, erect steel frames on the upper step, the left side of the middle step, and the right side of the lower step, install foot-locking anchor pipes, drill system anchor rods, and erect the inverted arch steel frame;

[0071] 3) Spray the initial support concrete of the invert and backfill the hole with slag;

[0072] 4) Re-spray concrete to the designed thickness on the upper step, the left side of the middle step, and the right side of the lower step, and spray 5cm of concrete on the face to seal it.

[0073] In step S3, the radial grouting range on the side of the middle sandwich soil column in the first excavated small hole is from the arch top close to the side of the later excavated small hole to the base of the invert arch close to the side of the later excavated small hole, and the end of the grouting pipe in the middle sandwich soil column range penetrates into the excavation surface of the later excavated small hole by not less than 50 cm; the arch frame spacing in the first excavated small hole is increased to 0.5m, and at the same time, a large arch foot is set at the bottom of the arch frame on the side of the middle sandwich soil column to enhance the soil deformation resistance and enhance the stability of the middle sandwich soil column.

[0074] Use φ42 (t=3.5mm) grouting pipes with a spacing of 1.2*1.0m or 1.2*1.2m (circumferential*vertical) and a plum blossom arrangement. The grouting material is mainly ordinary cement plus HPC admixture single-liquid slurry, supplemented by ordinary cement-water glass double-liquid slurry, which is adjusted in time according to the site conditions during grouting; the HPC admixture dosage is 10% to 15% of the cement mass, the water-cement ratio of ordinary Portland cement is 0.8:1 to 1:1, and the ratio of ordinary Portland cement slurry to water glass is 1:1 (volume ratio); the water glass concentration is 38°

[0075] / Be', modulus 2.4 ~ 2.8. Grouting is carried out from bottom to top and from outside to inside in interval construction order. The initial grouting pressure is 0.5 ~ 1.0MPa. During the grouting process, it is adjusted according to the injection amount of slurry. The grouting pressure reaches 1.2MPa and the pressure is maintained for more than 10 minutes before the hole is completed. During grouting, an orifice pipe should be set at the orifice. During the grouting process, orifice plugs should be set at adjacent grouting holes to ensure that grouting can be carried out while drilling to avoid slurry leakage.

[0076] In step S4, the compaction grouting of the soil column side in the later excavated small hole includes reserving a grouting pipe for the initial support of the later excavated small hole, and performing compaction grouting on the back of the initial support after the initial support of the later excavated small hole is completed; the compaction grouting range is from the spandrel close to the side of the first excavated small hole to the arch foot close to the side of the first excavated small hole; the grouting material is cement plus HPC admixture single liquid slurry;

[0077] The tension anchor locking initial support reinforcement includes that when the thickness of the middle soil column is less than 2m, the grouting pipe in the first excavated small hole is respectively welded to the arch frame in the first excavated small hole and the arch frame in the later excavated small hole by connecting steel plates, which plays the role of tension anchor.

[0078] The stability of the middle soil column is achieved and the impact of the left tunnel construction on the right tunnel is reduced.

[0079] The arch frame in the later excavated small hole is longitudinally connected with steel plates on the side close to the first excavated small hole.

[0080] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for constructing an ultra-small clearance double-hole separated loess tunnel, characterized in that: A temporary blocking structure is constructed at the end walls of the left and right small holes that are transformed from the large hole in the tunnel. The left and right small holes are excavated at an offset. When the left and right small holes are excavated, the middle earth column is supported. The small hole is excavated first, and radial grouting of the middle earth column side + large arch foot reinforcement + tunnel bottom grouting reinforcement is adopted. The small hole is excavated later, and the middle earth column side compaction grouting + tension anchor locking initial support reinforcement is adopted.

2. The method for constructing an ultra-small clearance double-hole separated loess tunnel according to claim 1 is characterized in that: It includes the following steps in order: Step S1: Perform initial support reinforcement of the tunnel before the left and right small tunnels enter the tunnel, including external reinforcement and internal reinforcement; external reinforcement includes treatment of surface gullies, surface caves, and surface cracks; internal reinforcement includes radial grouting and arch construction; Step S2: constructing a temporary blocking structure at the end walls of the left and right small tunnels of the tunnel; Step S3: First, dig a small hole for tunnel construction, excavate support construction, invert and filling construction, and secondary lining construction; during the excavation and support construction stage, radial grouting of the middle soil column side, reinforcement of the large arch foot, and grouting reinforcement of the tunnel bottom are carried out; Step S4: post-excavation of small holes for entry, excavation support construction, and lining construction; during the excavation support construction stage, side compaction grouting of the middle sandwich soil column and locking of the tension anchor rods for initial support reinforcement are performed; Step S5: dismantling the sleeve arch and constructing the secondary lining of the left and right small holes; Step S5: Construction of reinforced concrete structure of the end walls of the left and right small tunnels; Step S6: The left and right small holes are excavated synchronously.

3. The method for constructing a double-hole separated loess tunnel with ultra-small clearance according to claim 2 is characterized in that: In step S1, the surface gully treatment includes surface survey, gully replacement, laying of wire mesh, and mortar plastering; Surface cave management includes surface exploration, cave backfilling and compaction; Surface crack management includes surface exploration, crack excavation, crack replacement and compaction.

4. The method for constructing an ultra-small clearance double-hole separated loess tunnel according to claim 3 is characterized in that: In step S1, radial grouting during tunnel reinforcement includes: arranging grouting pipes along the side walls and arch top of the tunnel, reinforcing the arch ring 5m outside the designed excavation contour line of the tunnel, the length of the grouting pipe is 5-7m, and the grouting holes are arranged in a plum blossom shape around the pipe wall. The grouting material is cement / water glass double slurry, and the initial grouting pressure is 0.5-1.0MPa; The construction scope of the sleeve arch is within 9m outside the end wall of the tunnel where the large tunnel becomes a double tunnel. The arch frame spacing of the sleeve arch is 1.2m. When the arch frame is installed, φ42 locking foot anchor pipes are set, with two locking foot anchor pipes at each location, and the length of the locking foot anchor pipe is 4m; the longitudinal connection of the arch frame is welded with steel bars; after the sleeve arch is installed, C25 concrete is sprayed within 50cm before and after the arch frame, with a spraying thickness of 30cm.

5. The method for constructing a double-hole separated loess tunnel with ultra-small clearance according to claim 2 is characterized in that: In step S3, the radial grouting range on the side of the middle soil column in the first excavated small hole is from the arch top close to the side of the small hole to be excavated later to the base of the invert close to the side of the small hole to be excavated later, and the end of the grouting pipe in the middle soil column range penetrates into the excavation surface of the small hole to be excavated later by not less than 50 cm; the arch frame spacing in the first excavated small hole is increased to 0.5m, and a large arch foot is set at the bottom of the arch frame on the side of the middle soil column.

6. The method for constructing a double-hole separated loess tunnel with ultra-small clearance according to claim 5 is characterized in that: In step S4, the compaction grouting of the soil column side in the later excavated small hole includes reserving a grouting pipe for the initial support of the later excavated small hole, and performing compaction grouting on the back of the initial support after the initial support of the later excavated small hole is completed; the compaction grouting range is from the spandrel close to the side of the first excavated small hole to the arch foot close to the side of the first excavated small hole; the grouting material is cement plus HPC admixture single liquid slurry; The tension anchor locking initial support reinforcement includes that when the thickness of the middle soil column is less than 2m, the grouting pipe in the first excavated small hole is respectively welded to the arch frame in the first excavated small hole and the arch frame in the later excavated small hole by connecting steel plates, which plays the role of tension anchor.

7. The method for constructing a double-hole separated loess tunnel with ultra-small clearance according to claim 2 is characterized in that: In step S2, the temporary blocking structure for constructing the end walls of the left and right small holes in the tunnel includes the blocking structure outside the excavation range of the left and right small holes in the tunnel big hole and the supporting structure within the excavation range of the left and right small holes; along the tunnel excavation direction from inside to outside, the blocking structure includes the drainage system, the steel support structure, the reinforced anchor rod, the first steel mesh and the first shotcrete layer in sequence; the supporting structure includes the second shotcrete layer, the anchor rod and the second steel mesh.

8. The method for constructing a double-hole separated loess tunnel with ultra-small clearance according to claim 7 is characterized in that: The steel support structure includes an outer support ring on the contour line of the tunnel large hole and an inner support ring on the contour lines of the left and right small holes. The outer support ring and the inner support ring are connected by a steel frame. The outer support ring, the inner support ring and the steel frame are located in the same plane. The steel frame includes main vertical supports, transverse supports and secondary vertical supports. The main vertical supports are located between the left and right small holes. A group of locking foot anchor pipes are installed at the bottom of the main vertical supports. The locking foot anchor pipes are welded to the main vertical supports with L-shaped steel bars. The secondary vertical supports are symmetrically arranged on both sides of the main vertical supports. A complete transverse support is installed in each width section of the steel frame, and the transverse support is connected to the main vertical supports, secondary vertical supports, outer support rings or inner support rings.

9. The method for constructing a double-hole separated loess tunnel with ultra-small clearance according to claim 3 is characterized by: 3:7 lime soil is used for gully replacement and crack replacement, and M10 mortar is used for mortar finishing during surface gully treatment.

10. The method for constructing a double-hole separated loess tunnel with ultra-small clearance according to claim 6, characterized in that: The arch frame in the later excavated small hole is longitudinally connected with steel plates on the side close to the first excavated small hole.

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